avoid cracks in concrete slabs-on-grade

7/30/2019 Avoid Cracks in Concrete Slabs-On-grade

1/10

AVOID CRACKS IN CONCRETE SLABS-ON-GRADE

By Harvey Haynes-Consulting Concrete Engineer

Part 1 of 2 Types of Cracks and Their Causes

SOIL MOVEMENT

HeaveExpansive clay soils swell or heave with an increase in moisture content. Cracks in concrete

slabs caused by heave can be identified by vertical offsets, cracks running parallel to an exteriorwall, or cracks exhibiting an X-shaped pattern in a small room. It is difficult to distinguish cracks

caused by heave from those caused by settlement.

SettlementSlabs that have been overloaded or have weak soil support will settle due to soil consolidation.

The strength of the slab greatly depends on the strength of the soil support. A slab four inchesthick can be quite strong on firm soil, but may crack easily on weak soil. Cracks due tosettlement can appear as half-circle cracks at the edge of slabs, cracks raveling along the edges,

slabs breaking into small pieces (sections of one or two feet per side), or diagonal cracks across

corners of slabs.

THERMAL BEHAVIOR OF CONCRETE

Seasonal Temperature ChangeIf concrete is cast in the summer, it can experience a temperature decrease of 100 F by the

middle of winter. This decrease in temperature can cause a slab 100 feet long to contract about

3/4 of an inch. This contraction movement may crack the slab. Contraction joints are placed inslabs to encourage the cracks to occur at the joint locations. If the slab is cast in winter, then the

concrete could experience a 100 F increase in temperature by the summer. In this case, the slabwould expand about 3/4 of an inch. Expansion joints are required to allow the slab to expand, or

the slab may buckle.

Daily Temperature ChangeDaily temperature change causes a varying temperature through the thickness of the slab. The

sun heats the top surface, which causes the concrete near the top to expand, and the slab developsa hump shape where the middle is higher than the edges. If the top surface is colder than the

bottom, the slab will have a curled shape where the edges are higher than the middle.

Heat of HydrationHeat of hydration is heat internally generated by the concrete during the chemical process of

cement hydration. Hydration is the mechanism by which cement sets and then gains strength.

During the first night after concrete is cast, a common crack that occurs is caused by the

combination of heat of hydration and warm ambient temperatures on the day the concrete is cast.Typically concrete is cast in the morning, and by the afternoon a hot sun raises the temperature


2/10

of the concrete, especially near the top surface. Internally, concrete generates a considerable

amount of heat due to heat of hydration. By sunset of the first day, the concrete can be quite

warm, easily 120 F. Cool evening temperatures initially reduce the temperature at the topsurface, and this cooler concrete contracts. At this young age the concrete is quite weak, and the

contraction movement can crack the concrete. By the next morning, the slab is found cracked.

For this reason, contraction joints (also called control joints) must be installed the same day thatthe concrete is cast.

SHRINKAGE BEHAVIOR OF CONCRETE

Crazing CracksThese cracks appear at the very top surface layer of the slab where a thin layer of cement pastehas lost water too rapidly and cracked. These cracks are very fine and shallow.

Plastic Shrinkage CracksPlastic shrinkage cracks develop when too much water evaporates while the concrete is fresh, or

plastic in consistency. These cracks have a distinct form. They are quite wide at the surface, theirdepth into the slab is usually limited to about one to two inches, they range in length from about

six inches to five feet, they usually develop parallel to one another, and they don't run to theedges of the slab.

Drying Shrinkage CracksThese are the typical shrinkage cracks, which develop after the concrete is hard. They can appear

randomly across slabs or have a uniform pattern. These cracks also extend from re-entry corners.

If a cube four inches on a side were to dry over several months, each side would decrease in

width by about 0.002 inch. The entire cube will decrease in volume. When cubes are laid end to

end for a distance of 20 feet, the change in length would be about 1/8 inch. In a slab this gapwould be the crack width.

Why does the cube get smaller? Technically, there are two contributing reasons why moisture

loss causes shrinkage, and the explanations relate to two different void sizes within the concrete.

Concrete of residential home quality contains about 20 percent void volume. Part of this volume

is microscopic pores called capillary voids, which were created by the original mixing water.Smaller voids, called gel voids, exist in the concrete within the hydrated cement particles. When

water evaporates from the capillary voids, capillary forces develop which place the water in

tension. Therefore the solids are placed in compression, and shrinkage occurs. When water

within the gel voids evaporates, the hydrated cement particles become smaller and additionalshrinkage occurs.

If ambient conditions are at 100 percent relative humidity a concrete slab will not shrink. At 40percent relative humidity, and many months of drying time, a slab 100 feet long can shrink up to

3/4 inch. Let rain soak the slab for a couple of days and the concrete will swell such that a

permanent shortening of 1/8 to 1/4 inch exists.


3/10

Uniform drying of concrete does not occur in slabs-on-grade because only one face is exposed to

evaporation. A moisture gradient exists through the slab thickness where the top is drier than the

bottom, so shrinkage will be greater at the top. This condition results in the slab having a cuppedor curled shape. This shape can be explained by picturing the 4-inch cube, where the top of the

cube is dry and shrinks by 0.002 inch, while the bottom is moist and does not shrink. The cube

would have a wedged shape. Place these cubes tightly together, end-to-end, for a distance of 20feet and the slab will have a curled shape.

It is difficult to observe the slight curvature of curling, but measurements have been made onhighway pavement. For a slab 15 feet in length, measurements in the morning showed curling

where the edges of the slab picked up about 1/8 inch. In the afternoon, the slab essentially was

flat. The effect of the sun caused the top surface to expand, which countered the shrinkage, and

the slab leveled out. When night returned, the slab cooled and shrinkage again created the curledshape of the slab.

There are situations where curling is restrained, such as a slab joined to a perimeter footing.

When curling is restrained, the stresses on the top surface of the slab are greater than if the slabwas free to curl, so cracks occur sooner. Because curling cannot be prevented, drying shrinkage

cracks in concrete slabs can only be minimized, not prevented.

Part 2 of 2

Recommendations to Avoid Cracks

SOIL MOVEMENT

HeaveThe critical factor in preventing expansive clay soils from causing problems with heave is to be

certain that the clay soil is damp (above its optimum moisture content) before concrete is cast.Watering the clay may take days to get it damp. It is advisable to have a soils engineer provide a

letter stating that the clay is at proper moisture content.

Slabs, such as garage floors or walkways next to a house, should be free to float on top of the soil

as opposed to being tied into the house footing. Place isolation joint material between the slab

and building.

Use reinforcing bars to distribute and limit the width of cracks and prevent vertical offsets. As a

guide, use #3 bars at 18-inch spacing each way in 4-inch thick slabs, and #4 bars at 20-inchspacing in 5-inch slabs. Do not use welded wire mesh of size 6x6-W1.4xW1.4 (also known by its

old designation of 6x6-10x10). Mesh of size 6x6-W2.9xW2.9 is marginal, but acceptable.

SettlementCompacted base rock four inches deep will provide a uniform, firm support for the slab. If base

rock is not used, be sure that the subgrade (native soil) can provide good support even when thesubgrade is wet. Use a 5-inch thick slab, which is 50% stronger than a 4-inch thick slab in

flexural strength.


4/10

THERMAL BEHAVIOR OF CONCRETE

Seasonal Temperature ChangeTo minimize cracks, different types of joints are installed in slabs.

Contraction or Control Joints: These joints are installed by placing plastic inserts or grooves intothe fresh concrete to a depth of one-fourth the thickness of the slab. Saw cuts are also used, but

should be made the same day that the concrete is cast. Spacing and layout are discussed later in

the drying shrinkage section.

Expansion or Isolation Joints: These joints are of a compressible material, such as fiberboard,installed the full depth of the slab. For expansion joints, spacing is between 50 to 100 feet.

Construction Joints: These joints mark where new concrete abuts existing concrete. The joint can

function as a contraction joint when detailed using bond breaker on one concrete face, or whenusing keyways or smooth dowels greased on one side. Constructions joints will not function as

contraction joints when design details call for roughening one concrete face or havingreinforcing bars tie together the new and existing concrete.

Daily Temperature Change and Heat of HydrationTo minimize cracks that appear the morning after concrete is cast, install saw cut contractionjoints on the same day that concrete is cast. Grooves and inserts also are good joints because they

are installed while the concrete is fresh.

SHRINKAGE BEHAVIOR OF CONCRETE

Crazing Cracks

These cracks are prevented by proper finishing methods; use light tamping, do not overwork thesurface, and do not add dry cement to the surface to absorb bleed water. If necessary, bleed water

can be vacuumed or dragged off.

Plastic Shrinkage CracksWindy or hot weather is a sign of danger. Keep the temperature of the concrete as low aspossible by having transit mix trucks stand-by in the shade, or cool the mixing drum by spraying

water on the outside surface. Have a sufficient crew size available for rapid placement and

finishing. In extreme hot weather, avoid exposing young concrete to the hot part of the day bystarting to cast concrete in the late afternoon or early evening.

Delay evaporation of bleed water by spraying fog mist across the work area; or better, use anevaporation control coating sprayed on the fresh concrete after bullfloating. Store this material

on the job so it is available on windy or hot days.

Drying Shrinkage CracksMix Design: The more water used in making concrete, the greater the amount of shrinkage.

Hence, do not add water beyond the amount necessary for proper slump.


5/10

Use as large an aggregate size as possible. Concrete with 1-1/2 inch maximum size aggregate

will shrink less than concrete with 3/4-inch maximum size, and both of these will shrink

considerably less than pea gravel concrete, which uses a 3/8-inch maximum size aggregate.

Vapor Retarders: Vapor retarders are used primarily to stop ground moisture from moving up

through the concrete slab. Acceptable vapor retarders are:

a. Clean 3/4-inch drain rock, 4 to 6 inches deep

b. Plastic sheeting covered by 1 to 2 inches of clean sandc. Plastic sheeting of minimum 10-mil thickness

The preferred system is (a) and (b) together. System (c) can result in excessive cracks in the slab

if a high slump concrete is used. Therefore, use system (c) only with a low slump concrete (3-

inch maximum).

Contraction Joints: Use saw cuts, inserts, or grooves to install contraction joints. Make certain

that saw cuts are installed on the same day the concrete is cast.

Spacing of contraction joints is given in the following table:

Contraction Joint Spacing

For Slab Thickness Of:

4 inches 5 inches 6 inches

Interior Slab 16 feet 20 feet 24 feet

Exterior Slab 10 feet 13 feet 15 feet

Any slab cast in an open environment is an exterior slab, which means that tilt-up slabs areexterior slabs. Patios and walkways are usually 3.5 inches thick and they should have a

contraction joint spacing of around 6 feet. If pea gravel concrete is used, then reduce therecommended spacing in the table by 3 feet. The simplest and surest method to minimize

most cracks in slabs is to follow this guidance on contraction joint spacing.

The layout of joints is best when square sections are made. When rectangular sections are made,the length should not exceed 1.5 times the width. Re-entry corners must have contraction joints.

Reinforcement: Welded wire mesh of size 6x6-W1.4xW1.4 (or old designation of 6x6-10x10) is

the most common type of reinforcement in slabs-on-grade, and it is essentially useless. If welded

wire mesh is used, then use 6x6-W2.9xW2.9; however, this is still a small amount of steel andwill do little in crack control.


6/10

Slabs-on-grade do not have a code requirement for being reinforced; hence, they can be of

unreinforced concrete with cracks controlled by a close spacing of contraction joints.

Cracks can occur even if reinforcing steel is used. The reinforcement is to control crack width

and spacing. A recommended amount of steel (which only provides partial effectiveness in

controlling crack widths) is the amount given for shrinkage and temperature reinforcement forstructural concrete in the American Concrete Institute Building Code Requirements for

Structural Concrete, ACI 318-95, Section 7.12. This amount is equivalent to #3 bars at 18-inch

spacing in 4-inch thick slabs and #4 bars at 20-inch spacing in 5-inch thick slabs. Contractionjoints are still required, and the spacing can be larger than that for unreinforced slabs. To allow

these joints to properly function, it is advisable to cut every other bar crossing the joints.

To obtain maximum effect in controlling cracks by steel reinforcement, an amount of steel three

times that given above is required. These slabs are considered as continuously reinforced

concrete, and contraction joints can be omitted.

Reinforcing bars are good because they can be chaired. Place the bars in the top half of the slab.For slabs of 5 inches or thicker, locate the bars 2 inches below the top of the slab. Don't allow the

chairs, or dobbies, to settle in the sand cushion layer. The reinforcing bars need to be near the topof the slab to minimize the width of the cracks. Cracks are widest at the top because the greatest

shrinkage occurs at the top.

Steel fiber reinforcement works well to control cracks. The fibers are added to the transit mix

truck in quantities of at least 30 to 50 pounds per cubic yard of concrete. Normal placing and

finishing procedures are used. Contraction joints are required, but at a larger spacing. Steel fibersare highly recommended.

Polypropylene and nylon fibers do not provide any benefit in controlling drying shrinkagecracks, but they do have value in controlling plastic shrinkage cracks. These fibers are added to

the transit mix truck in quantities of about 1 to 1.5 pounds per cubic yard of concrete.

Curing: The objective of curing is to allow concrete to gain strength. To do this, the temperature

must be above 40 F and water must be present within the concrete for the cement to hydrate.

The best curing method is to flood concrete, but the most common method is to spray curingcompound on concrete. This method attempts to contain the mix water inside concrete, and it is

moderately effective. In windy or hot weather, the coverage rate specified by the manufacturer

should be increased by 1.5 times. A rough surface also requires more compound than a smooth

surface.

As long as water is held inside the concrete, drying shrinkage does not occur. Eventually the

concrete decreases in moisture content, and then shrinkage begins. Curing allows concrete togain strength; stronger concrete will crack less than poorly cured or weaker concrete.

SUMMARY OF RECOMMENDATIONS

Soil Movement


7/10

o Expansive clay soil must be damp before concrete is placed.

o The subgrade must provide fairly uniform and firm support for the slab, even when the

subgrade is wet. If not, use base rock.

Thermal Behavior of Concrete

o To accommodate seasonal temperature changes, install expansion joints to prevent slabfrom buckling, and install contraction joints (also called control joints) to minimizerandom cracks due to contraction.

o To avoid cracks that appear the day after casting slabs, install contraction joints on the

same day the concrete is cast.

Shrinkage Behavior of Concrete

o

To avoid crazing cracks, prevent excessive paste on the top surface of fresh concrete bylight tamping, do not overwork the surface, and do not apply dry cement to the topsurface.

o To avoid plastic shrinkage cracks, use an evaporation-control coating on fresh concrete

during windy or hot weather. Also, use polypropylene or nylon fiber reinforcement tominimize plastic shrinkage cracks.

o Do not add mix water to concrete beyond the amount for proper slump.

o Use concrete with 3/4-inch maximum size aggregate or larger. For pea gravel concrete,

use extra-close spacing of contraction joints.

o During hot weather, keep the temperature of the fresh concrete cool while in the transit

mix truck by having truck stand-by in the shade and spraying the outside of the drum

with water.o Space contraction joints properly (see table). This is the simplest and surest method to

minimize most cracks.

o Layout contraction joints for square sections. For rectangular sections, make the length

less than 1.5 times the width.

o Put contraction joints at re-entry corners.

o Unreinforced concrete slabs are acceptable, but be certain that contraction joints are

properly spaced.

o Don't use welded wire mesh of size 6x6-W1.4xW1.4. In its place, use mesh of size 6x6-

W2.9xW2.9 or larger.

o For reinforcement to provide a fair benefit in crack control, use #3 bars at 18-inch

spacing in slabs 4-inches thick, and #4 bars at 20-inch spacing in slabs 5-inches thick.

o Chair reinforcing bars and mesh so they are located in the top half of slabs.

o Use steel reinforcement to control shrinkage cracks; steel fibers are more effective than

bars.

o Spray curing compound on fresh concrete immediately after the final finishing operation.Check to be sure the coverage rate is equal or greater than the manufacturer's

recommendation and 1.5 times the recommendation in windy or hot weather.


8/10

ACKNOWLEDGEMENTS

Sincere thanks to the following individuals for reviewing the tape series: Professor Kumar Mehta

of the University of California at Berkeley, Professor Paulo Monteiro of the University ofCalifornia at Berkeley, Professor Lawrence Kahn of the Georgia Institute of Technology, Mr.

Wayne Ferree of TerraTech Inc., Mr. Ashok Kakade of Concrete Science, Mr. Donald Pearman

of Pearman Construction Inc., and Mr. Andrew Bardakos of R.H. Wehner Construction Co.

REFERENCES FOR FURTHER STUDY

Concrete: Structure, Properties and Materials, by P. Kumar Mehta and Paulo J. M. Monteiro,

Prentice Hall, New Jersey, 2nd Edition, 1993, pp. 548.

Properties of Concrete, by A.M. Neville, John Wiley & Sons Inc., New York, 7th Edition, 1996,

pp. 842.

ACI Committee 224, Control of Cracking in Concrete Structures, Manual of Concrete Practice,

Part 3, American Concrete Institute, Detroit, Michigan, published annually, pp. 42.

ACI Committee 302, Guide for Concrete Floor and Slab Construction, Manual of Concrete

Practice, Part 2, American Concrete Institute, Detroit, Michigan, published annually, pp. 46.

Concrete Floors on Ground, Portland Cement Association, Skokie, Illinois, 1983, pp. 36.

Designing Floor Slabs On Grade, by Boyd C. Ringo and Robert B. Anderson, The Aberdeen

Group, Addision, Illinois, 1992, pp.199.

This is the first of an every-other-month column on slabs, based on the bookDesigning FloorSlabs on Grade by Boyd C. Ringo and Robert B. Anderson. Bob Anderson is in the process of

updating this book and a third edition is due out later this year.

Mary Hurd's introduction to the book starts with seven questions:

How thick should the slab be?

How strong should the concrete be? Is reinforcement needed?

Where should the joints be placed?

Can adding fibers enhance the slab's performance?

When is post-tensioning appropriate?

What can be done to control cracking?


9/10

Although generally these may be considered decisions made by the designer, contractors should

understand the basics of slab design in order to be an involved partner in a project. Design

includes all of the decisions, specifications, and details made and documented beforeconstruction can begin. It is based on both the subgrade support and the concrete material. The

authors regard design as a two-step procedure: thickness selection is done by one of the

recognized design methods, then other features such as joint location and treatment andconstruction tolerances are determined.

This month, we will start at the bottom and look at what information about the supporting soil isneeded.

Introduction. A slab on grade cannot be designed without numerical values that come directly

from knowing what supports the slab. At the very least, a value is needed for the modulus of

subgrade reaction, commonly referred to as k; however, the grade support system is morecomplicated than is indicated by a single value. In addition to k, it is necessary to know the

properties of the underlying soil and available fill material. In other words, to design and

construct a quality slab on grade, one needs to know as much as possible about the grade systemthat supports that slab. The flow chart summarizes an orderly approach to obtaining thisinformation.

Working with a soils specialist. The first consideration of any slab on grade design should bethat of securing adequate geotechnical information. This should put the person responsible for

the floor design into the process at the very beginning of any planning, which must include site

considerations. When alternative sites are being evaluated for a project, soils conditions are often


10/10

a significant economic factor. The floor designer should be able to advise the owner as to what

soils information will be needed. He should do this along with the geotechnical engineer in order

to provide an optimum geotechnical report. Too often the team effort of floor designer andgeotechnical engineer is missing. This can lead either to costly overspending in obtaining soils

information or to unexpected construction overruns due to omissions or errors in initial

information. It must be emphasized that the slab on ground designer should be engaged eitherbefore or at the same time as the geotechnical firm.

Limit risk with insufficient information. The authors have found that in much routine slab ongrade design no soils information is available other than the floor designer's experience. This

experience is occasionally in the jobsite area, but frequently is not within that geographical area.

This situation often leads to relying on what previous experience dictated, such as 6 incheshas always worked or the soil is good. If forced into this situation, the designer mustprotect himself by stating on the construction drawings what assumptions were made inthe design process. The designer should also limit his liability by noting in writing therisks and possible consequences of inadequate soil information. Such steps not only

protect the floor designer and inform the client but often result in the client's favorablereconsideration in providing geotechnical backup.

avoid cracks in concrete slabs-on-grade

Documents